MOULDS. 171 



lation. The spores also show an extraordinary power of 

 resistance to low temperatures. 



The moulds differ in a marked degree in their sensibility to 

 poisons. Botrytis, for instance, is killed in a solution of form- 

 aldehyde four times weaker than that which destroys Peni- 

 cillium, a fungus possessing extraordinary power of resisting 

 many poisons. Thus, it will withstand a solution of mercuric 

 chloride sixteen times stronger than that which will kill 

 Botrytis. Botrytis, on the other hand, can withstand a solution 

 of a poisonous silver salt four times as strong at that which 

 kills Penicillium. 



The mould species described in the following pages have 

 been selected partly on account of their growth forms (repre- 

 senting types of development), and partly because of their 

 occurrence in the fermentation industry ; they belong to 

 different sections of the system of " higher fungi." 



The higher fungi are grouped together as Oomycetes (to 

 which belongs the fungus causing the potato-disease, Perono- 

 spora or Phytophthora) , Zygomycetes (to which Mucor belongs), 

 Ascomycetes (including Penicillium), and Basidiomycetes (to 

 which groups the mushrooms belong). There are in addition 

 a large number of fungi which are only known to us in their 

 conidial form, such as Oidium, Dematium, Monilia, and Clado- 

 sporium, which, it is assumed, represent a stage in the life his- 

 tory of some of the higher fungi, as has indeed been definitely 

 proved in certain cases. These are known as " fungi imperfecti," 

 a title which is not intended to be a reflection on the fungi, but 

 solely to define our lack of knowledge respecting them. 



The general features of the nutritive physiology of fungi 

 will now be dealt with. The several forms make very varied 

 demands on the chemical constitution of the substratum. Some 

 are able to absorb certain of the chemical elements as such, but 

 the majority assimilate them in the form of organic or inorganic 

 compounds. A large number of fungi are saprophytic, living 

 on decaying substances ; others adopt a parasitic mode of life, 

 and derive their nourishment from the bodies of living organ- 

 isms. In dealing with nutrition, we distinguish between the 

 process of assimilation, the conversion of the absorbed food 

 substance into body substance, and that of disassimilation, the 



